Method for transmitting a signal from a transmitter to a receiver in a power line communication network, transmitter, receiver, power line communication modem and power line communication system

ABSTRACT

A method for transmitting a signal from a transmitter over a channel to a receiver on a Power Line Network, wherein said signal is OFDM-modulated on a set of sub-carriers, is proposed, wherein an OFDM tonemap and an eigenbeamforming encoding matrix are determined based on a channel estimation for each sub-carrier, a tonemap feedback signal and an eigenbeamforming feedback signal are generated, which are descriptive of said OFDM tonemap and said eigenbeamforming encoding matrix, respectively, and transmitted to the transmitter. A corresponding receiver, a transmitter, a power line communication and a power line communication system are described as well.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims the benefit ofpriority under 35 U.S.C. §120 from U.S. application Ser. No. 12/145,992,filed Jun. 25, 2008, the entire contents of which is incorporated hereinby reference. U.S. application Ser. No. 12/145,992 is based upon andclaims the benefit of priority under 35 U.S.C. §119 from European PatentApplication No. 07014436.5, filed Jul. 23, 2007.

The invention relates to a method for transmitting a signal from atransmitter to a receiver in a powerline communication network and to acorresponding transmitter and a corresponding receiver. The inventionalso relates to a power line communication modem and a power linecommunication system.

BACKGROUND

Power line communication (PLC), also called Mains Communication, PowerLine Transmission (PLT), Broadband Powerline (BPL), Powerband or PowerLine Networking (PLN), is a term describing several different systemsfor using power distribution wires for simultaneous distribution ofdata. A carrier can communicate voice and data by superimposing ananalog signal over the standard 50 Hz or 60 Hz alternating current (AC).For indoor applications PLC equipment can use household electrical powerwiring as a transmission medium.

In order to increase the bandwidth of PLC systems it has been proposedto use Multiple Input-Multiple Output schemes (MIMO), which are knownfrom wireless communication.

It is an object of the invention to further increase the bandwidth ofPLC systems.

The object is solved by a method, a receiver, a transmitter, a powerline communication modem and a power line communication system.

Further embodiments are defined in the dependent claims.

Further details of the invention will become apparent from aconsideration of the drawings and ensuing description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows steps of one embodiment of the invention,

FIG. 2 shows steps of a further embodiment of the invention,

FIG. 3 a, 3 b show block diagrams of a receiver according to furtherembodiments of the invention,

FIG. 4 a., 4 b show block diagrams of a transmitter according to furtherembodiments of the invention,

FIG. 5 shows a block diagram of a power line communication systemaccording to a further embodiment of the invention,

FIG. 6 shows a block diagram of a power line communication systemaccording to a further embodiment of the invention,

FIG. 7 shows a block diagram of a transmitter according to a furtherembodiment of the invention,

FIG. 8 shows a block diagram of a receiver according to a furtherembodiment of the invention,

FIG. 9 a shows a circuit diagram for impedance modulating devices,

FIG. 9 b shows a schematic diagram of the time-dependence of thevoltage, when impedance modulating devices are present,

FIG. 9 c shows a schematic diagram of a voltage-time relation with partsof similar channel capacities to explain a further embodiment of theinvention,

FIG. 10 shows a schematic diagram of building a feedback signalaccording to a further embodiment of the invention.

DETAILED DESCRIPTION

In the following, embodiments of the invention are described. It isimportant to note that all described embodiments in the following andtheir properties and technical features may be combined in any way, i.e.there is no limitation that certain described embodiments, propertiesand technical features may not be combined with others.

In a step S100 an OFDM tonemap is derived from a channel estimation.This is done by transmitting a training sequence, which might also bereferred to as a test signal. The signal is OFDM (Orthogonal FrequencyDivision Multiplex)—modulated, i.e. a plurality of sub-carriers is usedfor transmitting the training sequence. OFDM is a multi-carriermodulation scheme, which uses a large number of closely spacedorthogonal sub-carriers. Each sub-carrier is modulated with aconventional modulation scheme (such as quadrature amplitude modulation(QAM)) at a low symbol rate, maintaining data rates similar toconventional single-carrier modulation schemes in the same bandwidth. Inpractice, OFDM signals are generated using the Fast Fourier transformalgorithm. The primary advantage of OFDM over single-carrier schemes isits ability to cope with severe channel conditions—for example,attenuation of high frequencies at a long copper wire, narrowbandinterference and frequency—selective fading due to multipath—withoutcomplex equalization filters. Channel equalization is simplified becauseOFDM may be viewed as using many slowly-modulated narrowband signalsrather than one rapidly-modulated wideband signal.

Due to a multipath PLC channel, symbols of the training sequence areattenuated in a frequency selective way. A receiver evaluates thedisturbed symbols of the training sequence and calculates the channelproperties in a step S104. Instead of the wording “channel estimation”similar wordings like “channel conditions” or “channel properties”mainly describe the same features. Measurements which describe thechannel estimation are, for instance, bit-error-rate (BER) orSignal-to-Noise-Ratio (SNR).

The OFDM tonemap describes the constellation, which can be used on thesingle sub carriers. Since not all sub-carriers are influenced bydisturbances like noise on the channel in the same way, the modulationschemes can be different for the different sub-carriers. With increasingSNR a higher modulation scheme might be chosen. This optimizes the bestpossible throughput for the current channel conditions. An OFDM withadaptable modulation schemes is also referred to as “adaptive OFDM”.

In a step S108 an eigenbeamforming encoding matrix is generated from thechannel conditions. When using Multiple-Input-Multiple-Output (MIMO)schemes on a power line network for transmitting signals from atransmitter to a receiver, there are different MIMO approaches. WithMIMO different goals can be achieved: On the one hand MIMO can obtain acapacity gain by sending different streams over different transmit ports(Spatial Multiplexing). On the other hand, MIMO can obtain a diversitygain to combat fading by sending replicas of each symbol over differenttransmit ports (space-time or space-frequency-codings), like inAlamouti-schemes.

From the channel estimation a channel matrix H can be derived by writingtransmission coefficients hij (i: number of feeding port, j: number ofreceiving port) in a matrix form (here for instance for i=2 feedingports and j=4 receiving ports):

$H = \begin{pmatrix}h_{11} & h_{12} \\h_{21} & h_{22} \\h_{31} & h_{32} \\h_{41} & h_{42}\end{pmatrix}$

Note, due to the multipath channel the H-matrix is different for eachsub-carrier in case of OFDM transmission.

The channel matrix H can be decomposed into 2 parallel and independentSingle Input-Single output (SISO) branches by the help of a singularvalue decomposition

H = UDV^(H) With $D = \begin{pmatrix}\sqrt{\lambda_{1}} & 0 \\0 & \sqrt{\lambda_{2}} \\0 & 0 \\0 & 0\end{pmatrix}$

λ_(i) are the eigenvalues of the “squared” channel matrix H·H^(H). U andV are unitary matrices, i.e. U⁻¹=U^(H) and V⁻¹=V^(H), respectively.Upper H indicates the Hermitian operator, which is the transposed andconjugate complex (*) of a given matrix.

A channel capacity can be calculated as the sum of two independent SISOchannels. For frequency selective channels, the available bandwidth isdivided into N equivalent sub bands (N=number of sub-carriers):

$C = {B\; \frac{1}{N}{\sum\limits_{i = 1}^{N}{\sum\limits_{\mu = 1}^{2}{\log_{2}{\det \left( {I_{N_{R}} + \frac{\lambda_{i,\mu}E_{S}}{n_{T}N_{0}}} \right)}\mspace{14mu} {bit}\text{/}s}}}}$

With I_(NR): n_(r)×n_(r) identity matrix, n_(r): number of receivingpaths, n_(t): number of transmitting paths, B: Channel bandwidth, E_(S):total average transmission energy, N₀: AWGN (Average White GaussianNoise) Level.

For Spatial Multiplexing, 2 different symbols are sent via two transmitports. Let s_(i) be the vector of the 2 symbols sent on the i-thsub-carrier, then the vector of the 4 received symbols of each receivingport and i-th sub-carrier is:

r _(i) =H _(i) s _(i)

The sent symbol vector s_(i) can be detected with a detection matrixW_(i) . W_(i) can be realized by applying either a zero-forcing (ZF)algorithm or the minimum mean squared error (MMSE) algorithm. For ZF thedetection matrix is the Moore-Penrose-Inverse of the channel matrix:

W _(i)=(H _(i hu H) H _(i))⁻¹ H _(i) ^(H)

Then the sent symbols can be recovered:

y _(i) =W _(i) H _(i) s _(i) =s _(i)

If the channel state information is available at the transmitter,Eigenbeamforming can be applied. The two transmit symbols of eachsub-carrier are multiplied with the matrix V_(i), which is derived fromthe single value decomposition (SVD) of the channel matrix Hi). If thedetection matrix W_(i) is taken as W_(i)=U_(i) ^(H) the decoded symbolsare obtained as follows:

y _(i) =U _(i) ^(H) H _(i) V _(i) s _(i) =U _(i) ^(H) U _(i) D _(i) V_(i) ^(H) V _(i) s _(i) =D _(i) s _(i)

Since D_(i) is a diagonal matrix, the channel is decomposed into twoparallel and independent paths.

Thus, from the channel estimation the Eigenbeamforming encoding matrix Vis generated.

Step 100 and step 108 might also be performed in an exchanged order,i.e. it is not important whether the eigenbeamforming encoding matrix orthe OFDM tone-map is determined first.

In a step S110 an eigenbeamforming feedback signal and a tonemapfeedback signal is determined. The corresponding feedback signalscomprise information about the generated Eigenbeamforming encodingmatrix V and the OFDM tone-map.

Thus, a combined usage of eigenbeamforming MIMO and adaptive OFDM forbidirectional PLC systems is proposed. The combination of OFDM andeigenbeamforming MIMO is well suited for the PLC channel with its strongfading effects and quasi-static behaviour. Since in adaptive OFDMsystems a feedback channel is already provided, this feedback can beeasily used for transmitting an information about the eigenbeamformingencoding matrix as well. The amount of feedback can be decreased withcompression schemes. Therefore, the Eigenbeamforming MIMO-scheme, whichis known to be the most effective with respect to throughput, can beeasily applied for PLC systems.

The eigenbeamforming feedback signal and the tonemap feedback signalsare transmitted from the receiver to the transmitter in a step S112.Afterwards the transmitter generates a payload signal,—i.e. a signalthat is not a test signal but a signal with information that should betransmitted to further devices in the power line network—and transmitsthe payload signal to the receiver.

In a further embodiment a feedback signal is generated based on theeigenbeamforming feedback signal and the tonemap feedback signal andtransmitted to the transmitter instead of separately transmitting theeigenbeamforming feedback signal and the tonemap feedback signal.

In a further embodiment the Eigenbeamforming decoding matrix isdetermined based on the channel estimation and payload data is decodedin the receiver based on the eigenbeamforming decoding matrix.

In order to reduce the data traffic on the power line network in afurther embodiment the feedback signal is compressed prior totransmitting the feedback signal to the transmitter. The generatedcompressed feedback signal is then transmitted to the transmitter. Thus,the needed bandwidth can be reduced.

In a further embodiment the compression is based on aLempel-Ziv-Markow-algorithm (LZMA), in particular an LZ77 based code.Such codes are available that are also used by current data compressionmethods (e.g. for zip-files) and very efficient without real-timerequirements. Since changes in a channel on a power line network occurquite seldom, the PLC-channel is “quasi-static”, such compressionschemes are well suited for power line communication.

For MIMO channels with a more dynamic behavior in a further embodiment acompression algorithm is used, which is less time-consuming. A possiblesolution is a Huffmann-code. The Huffmann-code can be optimizedaccording to the probability of the different symbols, so that symbolsthat are more often used get the shortest encoding sequence. In thisfurther embodiment dynamic changing coding trees are fed back to thetransmitter together with the feedback signal. Changes in the OFDMtonemap and the encoding matrix results in changing probabilities of thedifferent symbols, a new coding tree is generated in the receiver andsent back as part of the feedback signal to the transmitter. Thus, aso-called dynamic Huffmann-code is implemented.

In a further embodiment a set of predetermined encoding matrices isstored in the transmitter and in the receiver with correspondingidentifiers of said encoding matrices. Within the receiver arepresentative encoding matrix from said set of predetermined encodingmatrices is selected, wherein said representative encoding matrix issimilar to said eigenbeamforming matrix. Afterwards only the encodingmatrix index is sent back to the transmitter as part of the feedbacksignal and not the complete encoding matrix. A complete encoding matrixwould result in a large amount of feedback data, e.g. 1000*4*16bis=64000bits, if there are one thousand sub-carriers, 2 transmit paths used (2×2encoding matrix), with a matrix element resolution of 16 bits. Due tothe discrete set of encoding and related decoding matrices, the amountof feedback data can be reduced. The degradation due to the usage of theset of encoding matrices (instead of “perfect” encoding matrices) issufficiently low. In this case, the receiver only sends back to thetransmitter the identifier (or index) of the matrix pair that should beused for the encoding/decoding process.

Since the PLC channel is quasi-static, i.e. changes in the channelestimation or channel properties do not appear too often (e.g. byswitching on a light), in a further embodiment the amount of feedbackdata can further be reduced. The update frequency is dependent on thefrequency of changes of channel conditions. The feedback signal or thechannel estimation after receiving a further disturbed training sequenceare compared with the already determined channel estimation or with thealready sent feedback signal, and only in cases when they differ fromeach other, a corresponding new feedback signal is sent back to thetransmitter.

In typical PLC channels neighbored sub carriers often use the samemodulation constellation resulting from the tonemap and/or the same pairof en/decoding matrices for the Eigenbeamforming MIMO. So the feedbacksignal can be compressed significantly by transmitting a common part ofthe feedback signal for sub-carriers, which exhibit similar channelconditions, which result in the same tonemap and/or the same encodingmatrices.

In PLC networks often periodic or repetitive impedance changes occur dueto so-called impedance modulating devices. Mobile phone chargers andother charging devices contain circuitry that change the mainsimpedance, depending on a line cycle duration. These periodic impedancechanges have a dramatic influence to data transmission over a powerline. An impedance change during a transmitted data burst results inwrong channel equalization values after the impedance changes and causesnot correctable transmission errors. Therefore, a Medium Access Control(MAC) of a power line communication system tries to place the data burstin time intervals, where the impedance keeps stable. In a furtherembodiment of the invention time-dependent repetitive patterns of saidchannel estimation are determined, like the patterns resulting from saidimpedance modulating devices. The repetitive pattern is divided intoparts in which the channel estimation remains essentially the same andthe corresponding feedback signals are determined for each of the partsand transmitted back to the transmitter together with an indicator ofthe repetitive pattern. Such indicator describes the time-dependentbehavior of the channel estimation, e.g. the frequency and number ofparts of the repetitive pattern and the corresponding starting and/orend times. The transmitter uses this indicator together withcorresponding feedback signals to transmit the data bursts therebytaking into account the respective channel estimation. Since theconditions on the channel are known in advance due to the periodicnature of the impedance modulation, it is not necessary to transmit thefeedback signal in every case, but only in cases where there is not onlysuch an already known repetitive behavior, but some additional effect.Thus, the amount of feedback data can further be reduced.

In FIG. 2 a further embodiment of the method for transmitting signals ina power line network 200 is depicted. A first or transmitting power linemodem 202 is intending to transmit a message or payload to a second orreceiving power line modem 204. In a step S206 an initial data burstwith a training sequence is sent from the first modem 202 to the secondmodem 204. Within the second modem 204 the MIMO channels are estimatedin a step S208. Then in a step S210 from the estimated MIMO channelseigenvalues, encoding and decoding matrices are calculated. Also in astep S212 the adaptive OFDM tonemap is calculated. Derived feedback datais compressed and a length information is added in a step S214 and thefeedback data or feedback signal is send back to the transmitter in astep S216. Within the receiver 204 in a step S218 the adaptive OFDMtonemap is chosen for decoding in a step S218 and the correspondingdecoding eigenbeamforming matrix is selected in step S220. Within thetransmitter 202 the adaptive OFDM according to the signaled tonemap fromthe feedback signal is selected in a step S222 and the signaled encodingeigenbeamforming matrix is selected in a step S224. Afterwards thepayload or message is built accordingly and sent as data burst to thereceiver 204 in a step S226. Within the receiver 204 the OFDM tonemapand the decoding matrix are used to generate the original signal.

A schematic block diagram of a receiver 300 according to a furtherembodiment of the invention is shown in FIG. 3 a. The receiver 300comprises a receiving unit 302, including a plurality of OFDMdemodulators 303 for demodulating signals received via sub carriers ofan OFDM-based signal. The receiving unit 302 is connected to a channelestimation unit 304, which is configured to estimate the characteristicsof the power line network channel and is further configured to determinean OFDM tonemap and an eigenbeamforming encoding matrix based on saidchannel estimation for each sub carrier. A tonemap feedback signal isdetermined by said channel estimation unit 304, which is descriptive ofsaid OFDM tonemap and an eigenbeamforming feedback signal is determinedbased on said eigenbeamforming encoding matrix. The channel estimationunit 304 is connected to a transmitting unit 306, which is configured totransmit the feedback signal to the transmitter.

In a further embodiment the channel estimation unit 304 is furtherconfigured to generate a feedback signal based on said eigenbeamformingfeedback signal and said tonemap feedback signal, and said feedbacksignal is transmitted via the transmitting unit 306 to the transmitter.

A further embodiment of a receiver 300 is depicted in FIG. 3 b.Additionally the receiver comprises a de-eigenbeamforming unit 308,which is connected to the channel estimation unit 304 and aQAM-demodulation unit 310, which is connected to the de-eigenbeamformingunit 308. The de-eigenbeamforming unit 308 and the QAM-demodulation unit310 are used to re-generate the transmitted original signal. The channelestimation unit 304 is configured to generate an eigenbeamformingdecoding matrix, which corresponds to the generated eigenbeamformingencoding matrix and transmits the decoding matrix to thedeeigenbeamforming unit 308. The channel estimation unit 304 also sendsthe OFDM tonemap to the QAM-demodulation unit 310. In addition acompression unit 312 is connected between the channel estimation unit304 and the transmitting unit 306. The compression unit 312 isconfigured to compress the feedback signal prior to transmitting it inorder to reduce the bandwidth needed for the feedback.

In FIG. 4 a a schematic block diagram of a transmitter 400 is depicted.The transmitter comprises an adaptive QAM modulation unit 402, aneigenbeamforming unit 404, an OFDM modulation device 406 and a receivingdevice 408. A signal which is intended to be transmitted to a receiveris QAM modulated in the adaptive QAM modulation unit 402 according tothe latest OFDM tonemap. The eigenbeamforming unit 404 is connected tothe QAM modulation unit 402. Within the eigenbeamforming unit 404 theincoming symbols are multiplied with the eigenbeamforming encodingmatrix and the resulting symbols are transmitted via the OFDM modulationdevice 406. The receiving device 408 is connected to the adaptiveQAM-modulation unit 402 and to the eigenbeamforming unit 404. Thereceiving device 408 receives feedback data, i.e. the feedback signalfrom the receiver, and forwards the eigenbeamforming encoding matrix tothe eigenbeamforming unit 404 and forwards the OFDM tonemap to theadaptive QAM modulation unit 402.

In FIG. 4 b a schematic block diagram of a further embodiment of atransmitter 400 is depicted. Additionally a decompression unit 410 isconnected between the receiving device 408 and the adaptive QAMmodulation unit 402 and the eigenbeamforming unit 404, respectively. Thedecompression unit 410 is configured to decompress the feedback signal.

A schematic block diagram of a power line communication system or powerline network 500 is depicted in FIG. 5. A power line modem 502 in atransmission MIMO mode (Tx MIMO) with two transmitting paths 504 usesthe channels 506 of the power line communication system 500 and a powerline modem 508 in a receiving MIMO mode (Rx MIMO) receives the signal onfour receiving paths 510.

The two transmission paths result from two out of three possibilities tofeed signals on a home installation with three wires, i.e. phase line(P), neutral line (N) and protective earth (PE). Signals may be fedbetween phase and neutral (P-N), phase and protective earth (P-PE)) andneutral and protective earth (N-PE). According to Kirchhoff's rule thesum of the 3 input signals has to be equal to zero. Thus, only two outof the three possible input ports can be used. On the receiving side allthree differential reception ports can be used. Additionally a commonmode (CM) path can be used. Leakage current flowing between ground andearth due to AC (alternating current) primary or secondary neutralcurrents in the power-distribution system can produce a potentialdifference between the neutral and frame ground. Due to electromagneticcoupling between neighbored wires crosstalk arises, i.e. the transmitsignal from any feeding port is visible on all 4 reception paths.

In FIG. 6 a more detailed block diagram of a power line communicationsystem 600 is depicted. The first power line modem 502 and the secondpower line modem 508 may comprise both the functions of the receiver andthe transmitter, so that a bidirectional communication network is built.In direction A the second power line modem 508 receives data from thefirst power line modem 502. The receiving modem estimates the channel ina step S602, calculates the eigenbeamforming encoding and decodingmatrices as well as the adaptive OFDM tonemap for this direction. Thenthe receiving modem maps the calculated eigenbeamforming matrices to thenearest en-/decoding matrix pair of the available set of matrices(quantization). The related matrix pair index as well as the adaptiveOFDM tonemap are then processed in a way to reduce the amount offeedback data (e.g. data redundancy, compression). The compressed datais sent back from the second modem 508 to the first modem 502 over thePLC channel 506. The described procedure is performed for bothdirections separately. The illustrated number of wires is just forillustrating purposes. Both communication directions are separated inTime Division Multiplexing, i.e. in transmit mode the modem feeds intotwo differential feeding ports (three wires), while in receive mode ituses up to four receiving ports. Receive and transmit mode use of coursethe same set of available wires.

In FIG. 7 a more detailed block diagram is depicted for parts of afurther embodiment of a transmitter 400. The incoming data (data in) isencoded from a forward error correction state FEC and fed into thedifferent transmit paths (tx path 1, tx path 2) by a serial-to-parallelconverter 702. On each path an adaptive QAM modulator 704, 706 processesthe data. Afterwards the two paths enter the eigenbeamforming encodingunit 404, where the incoming adaptive QAM symbols are multiplied withthe eigenbeamforming encoding matrix. The encoded symbols are fed to theOFDM modulators 708, 710 of the different transmit paths.

In FIG. 8 a more detailed block diagram is depicted for parts of afurther embodiment of the receiver 300. The first stage of the receiverare different OFDM demodulators 802 (e.g. up to four for the PLCsystem). Then a channel equalizer within the channel estimation unit 304calculates the channel matrix of the MIMO channel. This channel matrixis used for the single value decomposition (SVD) that is used for theeigenbeamforming MIMO (eigenvalues and en-/decoding matrices).Afterwards the de-eigenbeamforming unit 308 performs matrix operationsto reduce back to the two different data paths. These two paths aredecoded by adaptive QAM demodulators 804, 806 before the data isassembled from a parallel-to-serial converter and a final FEC decodingstage 810 is applied.

Compared to other MIMO schemes as Alamouti or Spatial Multiplexing MIMO,Eigenbeamforming MIMO is considered as the most complex MIMO scheme. Onthe other side, it offers the best throughput performance.

The main difference is the need of sending feedback data from thereceiver to the transmitter: For each of the OFDM sub-carriers, anencoding matrix has to be send back to the transmitter. This results ina huge amount of data. There are several possibilities to reduce theoverall feedback datarate, e.g.:

a) In a dynamic channel, the feedback is needed for each burst. Instatic channels, the feedback information is just needed once. RegardingPLC, the channel is considered to be quasi-static. This means, feedbackinformation is just needed if there's a (seldom) change in the channelbehavior.

b) If several neighbored sub-carriers have the same channelcharacteristic, the system can bundle encoding matrix information ratherthan sending back matrix information for each sub-carrier.

c) One common set of pre-selected en-/decoding matrices is shared by allsubcarriers: This is a fundamental method to reduce the data: Not thematrix data itself is send back to the transmitter but the index of theen-/decoding matrix that should be used. Since the encoding process ontransmitter side is just a phase rotation, the degradation due to theusage of a set of encoding matrices (instead of the ‘perfect’ encodingmatrices) is sufficiently low.

In addition to the Eigenbeamforming MIMO, adaptive OFDM needs thefeedback of the sub-carrier modulation from the receiver to thetransmitter. Upper points a) and b) can be also applied to adaptiveOFDM: The sub-carrier modulation information is not needed for everysub-carrier and only if there's a significant change in the channelcharacteristics.

Therefore, the combination of adaptive OFDM and Eigenbeamforming MIMO iswell chosen for the PLC channel with its strong fading effects andquasistatic behavior. Since in case of an adaptive OFDM a feedback of atonemap is already used, the additional feedback for theEigenbeamforming encoding matrix may be easily implemented.

FIG. 9 a shows a circuit diagram and FIG. 9 b shows the correspondingtime-dependence of the voltage Ua on a power line, if impedancemodulating devices are present. Mobile phone chargers and other chargingdevices contain this circuity that has the following properties:

-   -   If the capacity C charges, HF-signals from Mains are shortcut,    -   If the diode is blocking, the rectifier has a high input        impedance,        So the mains impedance changes at least twice within the line        cycle duration.

The periodic impedance changes have dramatic influence to datatransmission over power line. An impedance change during a data burstresults in wrong channel equalization values after the impedance changeand causes non-correctable transmission errors. Therefore it isimportant to place the burst in time intervals where the impedance keepsstable, which is a task for a medium access control (MAC) layer of apower line communication system.

In FIG. 9 c it is depicted that depending on the line cycle frequency,different channel conditions result in different sets of en-/decodingmatrices (in this example: two different channel conditions, but moredifferent channel conditions might be possible as well). The y-axisrepresents the voltage of an AC line cycle.

By default, the complete information exchange has to be repeated foreach impedance change of the channel. Within this embodiment of theinvention in presence of impedance modulating devices the feedbackprocess is performed separately for each impedance condition:

Since the channel characteristics ‘toggle’ in a discrete way betweenseveral condition sets (depending on the line cycle frequency and thenumber of impedance modulating devices in the network), the receiver andtransmitter provide several sets of en-/decoding matrices that match tothe different available channel conditions. The MAC is responsible forplacing the data bursts completely within one phase without channelchanges. Therefore it is also within the responsibility of the MAC totell the receiver/transmitter which set of en-/decoding matrices shouldbe used for the Eigenbeamforming processing.

The proposed scheme can be used for all repetitive channel conditions(not only for impedance modulating devices). The proposed compressionscheme for data reduction is applied to each feedback processseparately.

In FIG. 10 the building of the feedback signal is depictedschematically. At the top for a plurality of sub-carriers 1 . . . n thechannel is estimated, resulting in different channel conditions for therespective sub-carriers, which is shown schematically by the differentlengths of the arrows. For each sub-carrier a constellation is chosenaccording to its current SNR value (the tonemap is the set of allsub-carrier modulation indices). In addition, the related en-/decodingmatrices for the eigenbeamforming are calculated. For the encoding anddecoding matrices the respective indices are selected from apredetermined set of matrices. A feedback signal 1000 is generated fromthe constellation tonemap and from the matrix indices for thesub-carriers. The feedback signal is compressed and a compressedfeedback signal 1002 is obtained and prefixed with its lengthinformation. The compressed feedback signal 1002 is sent back to thetransmitter.

Data compression algorithms for the feedback data are used in order toreduce the overall amount of data for each information feedbacktransfer. The compressed feedback information is expected to be veryeffective since the feedback data shows often repetitive data patterns:Neighbored (or even many neighbored) sub-carriers have the sameselection on sub-carrier constellation and en-/decoding matrices for theEigenbeamforming processing. Repetitive data patterns or data patternswith long constant symbol chains are most effective for compressionpatterns.

As mentioned, the PLC channel shows a quasistatic behavior: Changes inthe channel characteristic occur quite seldom. In this case, anefficient compression mechanism without real time requirements can beapplied. For the quasistatic PLC channel, state of the art LZ 77 basedcodes can be chosen for (de-) compression.

As an example, Lempel-Ziv-Markow-algorithms could be applied, that arepublic available compression schemes that are also used by current datacompression methods (e.g. zip).

The receiver compresses the tonemap and the encoding matrix indices, adda length prefix and sends it as feedback data back to the transmitter(described below).

For MIMO channels with a more dynamic behavior the decoding ofLempel-Ziv-Markow—algorithms might be too time consuming. In this casean algorithm that is more applicable to continuous data flows can beused. One possible solution is the usage of codes like the Huffmanncodes, as it is used in many digital standards.

In addition, the Huffmann encoding can be optimized according to theprobability of the different symbols—the idea is that symbols that areused most often get the shortest encoding sequence.

In this case dynamic changing coding trees are fed back to thetransmitter: Changes in the tonemap and the encoding matrix indicesresults in changing probabilities of the different symbols, a new codingtree is created in the receiver and must be sent as part of the feedbackdata back to the transmitter.

Thus, a combined usage of eigenbeamforming MIMO and adaptive OFDM forbidirectional PLC systems is proposed. The combination of OFDM andeigenbeamforming MIMO is well suited for the PLC channel with its strongfading effects and quasi-static behaviour. Since in adaptive OFDMsystems a feedback channel is already provided, this feedback can beeasily used for transmitting an information about the eigenbeamformingencoding matrix as well. The amount of feedback can be decreased withcompression schemes. Therefore, the Eigenbeamforming MIMO-scheme, whichis known to be the most effective with respect to throughput, can beeasily applied for PLC systems.

1. A transmitter for transmitting signals over a Power Line Network(PLN) to a receiver, said PLN including at least phase, neutral andprotective line wires so that said PLN establishes multiple wiredtransmission paths, said transmitter comprising: an adaptiveQAM-modulation unit; an eigenbeamforming unit connected to said adaptiveQAM-modulation unit; an OFDM modulation device connected to saideigenbeamforming unit; and a receiving device connected to said adaptiveQAM-modulation unit and to said eigenbeamforming unit, wherein saidreceiving device is configured to receive an eigenbeamforming feedbacksignal and a tonemap feedback signal from a receiver via said PLN, saideigenbeamforming feedback signal being descriptive of aneigenbeamforming encoding matrix and said tonemap feedback signal beingdescriptive of an OFDM tonemap corresponding to said multiple wiredtransmission paths of said PLN, and wherein said adaptive QAM-modulationunit is configured to use said OFDM tonemap and said eigenbeamformingunit is configured to use said eigenbeamforming encoding matrix.
 2. Thetransmitter according to claim 1, further comprising: a decompressionunit, connected between said receiving device and said adaptiveQAM-modulation unit and between said receiver device and saideigenbeamforming unit, said decompression unit being configured todecompress said feedback signal.
 3. The transmitter according to claim2, wherein said decompression unit is configured to decompress saidfeedback signal based on a Lempel-Ziv-Markov-algorithm based on an LZ77based code.
 4. The transmitter according to claim 2, wherein saiddecompression unit is configured to decompress said feedback signalbased on a Huffmann-code.
 5. The transmitter according to claim 4,wherein a Huffmann-code optimized coding tree is based on theprobabilities of symbols, which are expected to be used within saidtransmitter with said OFDM tonemap and said eigenbeamforming encodingmatrix.
 6. The transmitter according to claim 1, wherein saiddecompression unit is further configured to extract a length informationencoded in said compressed feedback signal.
 7. The transmitter accordingto claim 1, wherein: said feedback signal is common for differentsub-carriers when said eigenbeamforming encoding matrices for saiddifferent sub-carriers are the same.
 8. The transmitter according toclaim 1, wherein: said feedback signal is common for differentsub-carriers when said OFDM tonemaps for said different sub-carriers arethe same.
 9. The transmitter according to claim 1, wherein: saidfeedback signal includes an indicator of a repetitive pattern obtainedby determining time-dependent repetitive patterns of a channelestimation, by dividing said repetitive pattern of said channelestimation into parts, wherein within said parts said channel estimationremains essentially the same, and by generating corresponding feedbacksignals for each of said parts, and using said indicator together withthe corresponding feedback signal to transmit data bursts thereby takinginto account the respective channel estimation.
 10. The transmitteraccording to claim 1, wherein: a set of predetermined decoding matricesis stored in the transmitter and the feedback signal contains anidentifier identifying one of the decoding matrices matching with thecannel estimation.